This invention relates to a heat-sensitive stencil, to a process of fabricating same and to a printer using same.
One known heat-sensitive stencil is composed of an ink-permeable thin paper serving as an ink support and a thermoplastic resin film bonded with an adhesive to the support. The stencil is heated imagewise by, for example, a thermal head to perforate the heated portions of the thermoplastic resin film, thereby obtaining a printing master for reproducing images by mimeographic printing. The conventional stencil, however, poses problems because (1) the adhesive tends to be accumulated in interstices between fibers to form “fins” which prevent the thermal perforation during the master forming step and the passage of an ink during the printing step, (2) the fibers per se prevent smooth passage of an ink and (3) the paper support is relatively expensive.
To cope with the above problems, JP-A-54-33117 proposes a stencil having no paper support and composed substantially only of a thermoplastic resin film. While this stencil can completely solve the above-mentioned problems, a new serious problem arises; i.e. it is necessary to significantly increase the thickness of the stencil in order to obtain satisfactory stiffness required for transferring the stencil master during printing stage. An increase of the thickness results in the lowering of the thermal sensitivity.
U.S. Pat. No. 5,843,560 discloses a heat-sensitive stencil having a porous resin layer formed on a thermoplastic resin film. The stencil is produced by applying, on a surface of the thermoplastic resin film, a coating liquid containing a resin dissolved in a mixed solvent of a good solvent capable of dissolving the resin and a poor solvent substantially incapable of dissolving the resin and having a lower evaporation rate than the first solvent. The applied coating liquid is then heated to dryness. During the course of the evaporation of the solvents, pores are formed. This stencil has been found to be able to solve the above-described problems but to because a new problem that the porous resin layer is separated from the thermoplastic resin film. For example, when a printing master obtained from the stencil is removed from the plate cylinder after having been used for producing a large number of prints, only the thermoplastic resin film tends to be removed therefrom with the porous resin layer remaining thereon. Additionally, the known stencil fails to exhibit sufficiently high stiffness in humid conditions so that the transferability thereof in the printer is not fully satisfactory.